In electrophotographic apparatus, such as photocopiers, magnetic brushes are used to transport two-component developers. Two-component developers consist essentially of mixtures of toner particles and carrier particles. The toner particles are pigmented, resinous, non-magnetic particles. The carrier particles are magnetic particles, such as ferrite particles.
Typically, a magnetic brush comprises an outer sleeve, which is tubular, and an inner core, which is cylindrical. The inner core is disposed within the outer sleeve to permit relative rotation of the inner core and the outer sleeve about a common axis. The inner core and the outer sleeve may be independently rotatable in opposite rotational senses or in identical rotational senses. The outer sleeve is made of non-magnetic metal, which may be electrically conductive, such as non-magnetic stainless steel. The inner core is magnetized so as to exhibit a magnetic field with alternate north and south poles around the magnetic core.
The magnetic field exhibited by the inner core attracts the carrier particles to the outer surface of the outer sleeve. Opposite rotation of the outer sleeve and the inner core produces triboelectric forces, which cause the toner particles to adhere to the carrier particles. Such rotation also transports the carrier and toner particles around the outer surface of the outer sleeve, until the toner particles are stripped from the carrier particles, as by an electrostatic image on an image-recording surface near the outer sleeve.
A magnetic brush may be similarly used to transport a one-component developer. A one-component developer consists essentially of magnetic toner particles.
Miskinis et al. U.S. Pat. No. 4,546,060 illustrates and describes a magnetic brush and its operation with two-component developers. See, also, Paranjpe et al., A Magnetomechanical Model of the Magnetic Brush, J. Appl. Phys. 63(6), pp. 2136-2140 (1968).
In a magnetic brush of one known form in commercial use heretofore, the inner core comprises a shaft and a plurality of ceramic bar magnets. The magnets which are mounted adhesively along the shaft to exhibit a magnetic field with alternate north and south poles about the magnetic core.
Such a brush with ceramic bar magnets is bulky, and heavy, and requires motors with high torque. Fabrication of its inner core tends to be very inefficient. Such a brush cannot be easily accommodated in designing a compact unit of electrophotographic apparatus, such as a photocopier for desk-top use.
There has been a need, to which this invention is addressed, for a magnetic brush in an improved form, which can be compactly and efficiently made.